The present invention relates to a process for producing light-weight concrete products and more particularly it relates to a process for producing a light-weight aggregate for use in the production of light-weight structural products.
Approximately 3/4 of the volume of conventional concrete is occupied by aggregates consisting of such materials as sand, gravel, crushed rock, or air-cooled blast-furnace slag. It is inevitable that a constituent occupying such a large percentage of the mass should contribute important properties to both the plastic and hardened product. Additionally, in order to develop special light-weight, thermal and acoustical characteristics, aggregates manufactures specifically to develop these properties in concrete are employed. These aggregates may occupy less than 3/4 of the volume in structural concrete and more than 3/4 of the volume in masonry units and other applications.
When concrete is freshly mixed, the aggregates really are suspended in the cement-water-air bubble paste. Behavior of this suspension (i.e., the fresh, plastic concrete), for instance, ease of placement without segregation causing rock pockets or sand streaks, is importantly influenced by selection of the amount, type and size gradation of the aggregate. Depending upon the nature of the aggregates employed, a fairly precise balance between the amount of fine-and-course-sized fractions may have to be maintained to achieve the desired mobility, plasticity, and freedom from segregation, all lumped under the general term "workability." Selection of mixture proportions is aimed to achieve optimum behavior of the desired properties.
Aggregates contribute many qualities to the hardened concrete. The strength-giving, binding material holding concrete together results from the chemical union of the mixing water and cement and is, of course, the basic ingredient. However, this hardened cement-water-air bubble paste would, by itself, be a very unsatisfactory building material due to its high cost and volume shrinkage. Unless restrained by contained aggregates, the paste undergoes an intolerable amount of shrinkage upon drying. The exposed portions of such pastes dry out first, and differential shrinkage between the outside and inside portions often results in cracking. The presence of aggregates provides an enormous contact area for intimate bond between the paste and aggregate surfaces. Rigidity of the aggregates greatly restrains volume change of the whole mass. The aggregates should also contribute to workability, volume, stability, unit weight, resistance to destructive environment, strength, and present an acceptable surface texture.
Satisfactory concrete has been made with aggregate consisting of particles of a great variety of individual shapes. Natural aggregate particles which have been subjected to wave and water action over geologic history may be essentially spherical; other broken by crushing may be cubical or highly angular with sharp corners. Of intereset to the concrete technologist is that such changes in shape will be influential in altering the void characteristics of the aggregate. A highly angular coarse aggregate possessing a large void content will demand a greater amount of sand to provide a workable concrete. Conversely, a well-rounded course aggregate tending toward spherical particles will require less sand. Thus a rounded aggregate may contribute to better workability in a concrete mix. It is interesting to note, however, that concretes made with a great disparity in particle shapes at a given cement content will frequently have about the same compressive strength.
As such, it would be desirable to have a light-weight (less than about 60-70 lb/ft.sup.3), nearly spherical aggregate compatible with concrete which still retains all of the strength characteristics of natural aggregate particles.
"Lightweight aggregates" of various types have been used for many years, but it is only in the last twenty-five years that they have become an important factor in the concrete industry. Both mineral (inorganic) and vegetable (organic) materials of many varieties have been considered and tested for use as light-weight aggregates. The inorganic or mineral materials are the only ones that have been used to any significant extent.
Various methods have been used to classify light-weight aggregates into general types. These are frequently based on original source as either natural or artificial materials. The artificial aggregates are either by-products of an industrial operation or specially produced by calcining, expanding or sintering. In this classification system, such materials as pumice, scoria, and tuff are natural aggregates. By-product materials include cinders, bottom-ash, and processed by-products. Processed light-weight aggregates may also be produced from clay, shale, slate, perlite, vermiculite, blast furnace slag, and fly ash.
Some of the above materials may be placed in more than one of the groups. For example, pumice may be used either in its original natural state or after heat processing (calcining). Blast furnace slag and fly ash are by-products that have heretofore been processed for use as light-weight aggregates by expanding and sintering, respectively. Clays, shales, and slates are natural materials that are always heat processed during production of light-weight aggregates.
There are basically three A.S.T.M. (American Society for Testing Materials) specifications that cover light-weight aggregates. These specifications are as follows:
1. A.S.T.M. C-330, "Lightweight Aggregates for Structural Concrete" PA0 2. A.S.T.M. C-331, "Lightweight Aggregates for Concrete Masonry Units" PA0 3. A.S.T.M. C-332, "Lightweight Aggregates for Insulating Concrete
Of course, it is known that use of such light-weight aggregates, either natural or artificial, produces a "light-weight concrete." A light-weight concrete is a concrete lighter in weight than that usually obtained with "normal weight" aggregates (gravel and crushed stone). Light-weight concrete may be produced in the same manner as conventional concrete except for the use of light-weight aggregates.
The use of fly ash to make light-weight aggregates is known. For example, Minnick, U.S. Pat. No. 2,987,411, discloses a process for making a light-weight aggregate wherein fly-ash is initially mixed with a material such as clay, shale and mixtures thereof. The resulting mixture is then fired in a substantially non-oxidizing atmosphere at temperatures of from about 1500.degree. to 2400.degree. F.
It is also known that light-weight concrete may be produced by adding other light-weight materials such as light-weight structural foam. See, for example, U.S. Pat. Nos. 4,373,955; 4,097,422; 3,983,081; and 3,758,319, which discloses various types of light-weight foamed concretes.
It is further known that various agents can be introduced into a concrete mixture to achieve certain desired properties. For example, Sommer, U.S. Pat. No. 3,141,857 relates to a cellular concrete which is produced by agitating Portland cement with water and a foaming agent (which may be sulfonate containing) and then adding a mineral filler such as sand, gravel or crushed rock. Gilliot et al, U.S. Pat. No. 4,426,456 relates to an improved sulphur concrete which entrains air cells or microspheres in the material to improve its freeze/thaw characteristics. Gilliot discloses that a sulfate surfactant may be used as a gas entraining agent to modify the surface tension of the molten sulphur component of the sulphur concrete. Miller, U.S. Pat. No. 4,501,830 relates to a rapid set light-weight cement product which is produced from a mixture of cement, condensed silica fume, fly ash cenospheres, finely divided silica particles, epoxy emulsion, curing agent, accelerator and water. The silica fume used by Miller can be of the type disclosed by Cornwell et al in U.S. Pat. No. 4,321,243.
Finally, Kraemer Pat. Nos. 4,142,910 and 3,814,614 disclose light construction concretes including light-weight inorganic admixtures compatible with concrete. These admixtures are foamed or bloated to be approximately spherical or ellipsoidal shaped. The admixtures of the Kramer patents include foamed alkali metal silicate glass, foam glass granulates, and expanded or bloated clay. The Kraemer patents, among others, are discussed in the chapter entitled, "Lightweight and Foam Products" in Cement and Mortar Technology and Additives, Noyes Data Corp., Park Ridge, N.J. 1980.
While the admixtures of Kraemer have the advantage of producing a light-weight concrete having a dry density of less than about 0.9 kg/dm.sup.3 and a 28-day compressive strength of up to about 120 kg/cm.sup.2, the foaming or blowing procedure for such inorganic glasses is difficult, requires high temperatures, and can be costly.
Accordingly, the need exists for a more efficient means for producing a lower cost light-weight aggregate containing fly ash.